Printers are useful for producing printed images of a wide range of types. Printers print on receivers (or “imaging substrates”), such as pieces or sheets of paper or other planar media, glass, fabric, metal, or other objects. Printers typically operate using subtractive color: a substantially reflective receiver is overcoated image-wise with cyan (C), magenta (M), yellow (Y), black (K), and other colorants. Various schemes can be used to process images to be printed.
Printers with optical printheads expose engine pixels, specific areas on a photosensitive receiver, with specific engine pixel levels to form an image. However, printers can produce images exhibiting nonuniformity in the cross-track (X) and in-track (Y) directions. Visible nonuniformities that extend along the in-track direction are referred to herein as “streaks,” and nonuniformities that extend along the cross-track direction are referred to as “bands.” Nonuniformity in various printer systems can cause streaks and bands. For example, differences between the output powers of adjacent LEDs in an exposure system can cause streaks, and eccentricity of rollers in toning stations can cause bands.
In halftoned or multitoned screening, multiple adjacent engine pixels are grouped into a screen cell. In binary halftoning, each engine pixel is either exposed or not, and colorant is applied to the exposed pixels to form the image. The density of the halftone dot is therefore proportional to the number of engine pixels exposed in the screen cell. Multitoned systems, in contrast, provide more than two levels of exposure for each engine pixel. For example, an eight-bit system provides an unexposed level, or one of 255 progressively-increasing levels of exposure, which correspond to 256 possible density levels of each engine pixel. The number of engine pixels exposed in the screen cell for a given density level, and the individual exposures of those pixels, are selected to produce a pleasing tonescale in a multitoned image.
Various schemes exist for providing compensation for one-dimensional macro non-uniformity, referred to herein as streaking (extending in-track) or banding (extending cross-track). For example, U.S. Pat. No. 6,819,352 to Mizes et al. describes printing a test target, scanning it, determining nonuniformities, and adjusting drive current of an LED to compensate. U.S. Patent Publication No. 20060001911 by Viassolo et al. describes a method for compensating for streak defects in an image formed using a raster output scanning device by adjusting the intensity of exposure. This scheme includes generating a reflectance profile from an image generated by the raster output scanning device; determining a difference profile based upon the generated reflectance profile and a uniform profile; and generating a compensation parameter based on the determined difference profile, the compensation parameter representing a change in an intensity setting profile for the raster output scanning device.